Physics: Principles and Problems

Chapter 9: Momentum and Its Conservation

Problem of the Week

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Deep Space 1
Part of the new NASA fleet of "cheaper, faster, better" spacecraft, Deep Space 1, (DS1) is set to be launched in October of 1998. Compare DS1 with the Cassini spacecraft launched a year earlier in October of 1997:

DS1         Cassini
height x width (ft.)8 x 622 x 13
weight (lbs.)108012 800
cost (US$)$141 million$2.7 billion

Part of the savings is due to the revolutionary propulsion system. Basic physics still requires an engine that throws mass out the back and then according to conservation of momentum, the craft goes forward. Instead of the standard method of throwing gas exhaust out the back, DS1 throws xenon ions.

With Ion Propulsion, there is no roar from the engine, just an eerie blue glow.

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Ion Propulsion Engine
Ion propulsion has many advantages. For one, it is 10 times more efficient than conventional chemical propellants, so DS1 saves on weight. However, the DS1 only produces one eightieth of a pound of thrust. Even though the ions are fired out the back at 70 000 miles per hour, xenon ions are just not very massive. As of now, the JPL (Jet Propulsion Lab) scientists are expecting ion propulsion to power many missions in the new millennium.
<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078458137/193800/POWproblem1.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (0.0K)</a><a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg::::/sites/dl/free/0078458137/193800/ch09_ds1sm.jpg','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (8.0K)</a> In order to catch the West Kohutek-Ikemura comet and the McAuliffe astroid, DS1 has to attain a velocity of 22 000 mph. How long will it take DS1 to reach this velocity?
 
<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078458137/193800/POWproblem2.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (0.0K)</a> How many xenon atoms per second are exiting the ion propulsion engine at maximum impulse power?
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